1. Field of the Invention
The present invention relates to degradable netting, composites made with the degradable netting, and methods for making the same.
2. Background Art
The continuous extrusion of plastic netting started in the 1950's with the process described in U.S. Pat. No. 2,919,467 to Mercer. Extruded netting is netting in which the strands are extruded from a die, the joints being formed either within the die or immediately outside the die. A variety of configurations are known, such as square, diamond, twill, etc. Some of the more common materials used to prepare extruded netting are polypropylene, polyethylene (including very low, low, medium, high, linear grades, and ethylene copolymers), nylon, polybutylene, and blends thereof.
Currently, the extrusion process of choice for manufacturing plastic nets is one in which individual plastic strands are extruded in an interconnecting network to provide the net-like structure. Methods for practicing this technique are well known. For instance, U.S. Pat. No. 3,700,521; U.S. Pat. No. 3,767,353; U.S. Pat. No. 3,723,218; U.S. Pat. No. 4,123,491; U.S. Pat. No. 4,152,479 and U.S. Pat. No. 4,190,692 show apparatus and method for making nets by the continuous extrusion of individual plastic strands. The disclosures of the above-mentioned issued patents are incorporated by reference into the present application.
These nets have found a number of uses in commerce. For example, these nets have found use as agricultural netting, such as turf netting, turf wrap, hay bail wrap, erosion control netting, packaging netting, such as for onion and turkey bags, and netting for industrial, filtration and home furnishings applications.
Turf netting and turf wrap are examples of common applications for extruded nettings. Turf netting is durable mesh netting that is laid down in fields just prior to or after seeding to facilitate grass root consolidation. The netting allows the grass seedlings to germinate and grow while the roots intertwine with the durable mesh netting resulting in a uniformly strong structure. Because of the stronger root system, growers are allowed to harvest earlier with generally thinner slabs maximizing land utilization. Turf wrap is applied during harvest to reinforce large rolls of turf. The rolls of turf are wrapped to protect the turf during installation, to minimize turf loss, and to maintain roll quality. Wrapping rolls of turf also reduces loss during transportation and helps to protect against the weather. Turf wrap can remain on the rolls or be removed during installation based upon the customer's needs.
Netting has also found use in certain composites. In such composites the netting is laminated to one or more fabric overlays. Chief among such uses and composites are fabrics for disposable diapers, incontinent briefs, training pants, bandages, dressings, diaper holders and liners and feminine hygiene garments, medical gowns, medical drapes, mattress pads, blankets, sheets, clothing, consumer wipes and other like products, such as building and construction composites.
Since netting materials often find their way into the environment, either through their implanting as a result of their intended use or as waste or debris, it has become desirable to provide netting which is degradable. A degradable plastic material is defined, according to ASTM D20.96, as plastic material that undergoes a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification. Degradation can take place by exposure to heat, microorganisms, moisture, oxidation, UV light, other chemical reactions, and combinations thereof. Initially, plastic will degrade into smaller molecules as its components molecular weights decrease. This results in decreased mechanical properties of the plastic materials. Such decreases include lower tensile strength and increased brittleness. Degradation tends to result in the plastic material completely being broken up into smaller plastic particles.
In addition to being degradable, the netting must be extrudable. In certain, more common, extruded netting manufacturing processes, plastic netting is typically extruded though an annular die and quenched in a water tank. The extrusion typically takes a tubular form. The resulting tubular netting is collapsed in a quench tank, slit, and opened up to a flat sheet. The flat sheet is then wound onto rolls. This flat sheet netting is defined as “Stage 1 netting.”
To be compatible with this type of web handling system, the polymer material used in the extruded product must withstand transformation from an annular tube to a collapsed tube. The material must be flexible enough that this transition does not cause any permanent damage to the netting. The netting's folding point is particularly sensitive to damage.
After being wound, the flat sheet undergoes an orientation process where it is expanded, or stretched, in one or two directions—the machine direction and/or the cross-machine direction. The polymeric material used to form the netting must also be able to withstand this orientation process. Problems tend to occur during orientation when the polymeric materials are a blend of insufficiently homogeneous components. This can cause the netting to rip during the orientation process or result in netting that is unsatisfactorily weak. This flat sheet netting is defined as “Stage 2 netting.”
Polyolefins have been found to be a suitable material for use in the extruded netting manufacturing process. However, due to the sensitivities of the process, such as those of the orientation process, the use of even small amounts of seemingly acceptable additives to the polyolefin can render the resulting composition useless.
For instance, starch/polyolefin blends that have been used to form degradable plastic products, have been found not to be compatible with the extruded netting manufacturing process. Poor dispersion of starch, coupled with the large particle-size of starch, results in excessive interference with the molecular alignment process of the polyolefin in the orientation process. This causes a weakened product, or a product break in the orientation process.
Also, the polymer blend must be capable of being processed at relatively high temperatures, such as above 275° F. (125° C.), and above 400° F. (205° C.) in the case of some polyolefins, such as polypropylene. Problems that could occur if the material cannot withstand the processing temperatures include, degradation, bubbling, void formation and chemical breakdown of material.
Furthermore, the rate of degradation of any netting must be slow enough that the netting does not degrade (either at all or too much) before it has fully served its purpose.
As such, the material used to make the netting must be able to be extrudable to form netting having desired structural properties, such as flexibility, orientability, tensile strength and degradability, and be cost effective.